The present invention relates to preventing data recorded on a recording medium from being illegally copied and, more particularly, to preventing illegal copying of the recorded data by encoding key data in legally reproduced copies.
The private ownership of video tape recorders has spread so extensively in recent years that copying of video programs has become commonplace. Video rental stores rent video tapes to consumers for home entertainment and some consumers, without authorization, copy the video programs recorded on the rental video tapes. This problem is compounded by professional copiers, or pirates, who illegally copy the video programs en masse to distribute the illegal copies for profit.
While measures have been taken to prevent the illegal copying of video programs from rental video tapes, the problem is not so severe because copied video programs have poor image quality. This is because illegally copied video programs are originally recorded as analog signals, which do not lend themselves to accurate copying. The image quality of such an analog signal after copying several times, i.e., after several generations, is unavoidably deteriorated. For this reason, the duplication of copies is limited and the problem of copying analog video programs is somewhat manageable.
However, with the recent introduction of digital video recorders which have the capability of high quality copying, the problem of illegal copying is severe. Digital video, unlike its analog predecessor, has the advantage of being reproduced with high image quality and can be copied a number of times without deterioration. Accordingly, the problem of illegal copying is much more severe in the case of digital video recording and the prevention thereof is quite important to the digital video industry.
Heretofore, proposals to prevent the illegal copying of digital video programs have proven insufficient to thwart illegal copying. For example, it has been proposed to record a flag onto legal copies of the digital video program in the flyback period, indicating to a digital video recorder that the digital video program is a copy and should not be copied illegally. Nevertheless, professional copiers have easily circumvented this anti-copying technique by constructing special digital circuits and software which ignore the flyback period and illegally copy the digital video program. Since the flyback period does not contain a significant part of the digital video program, the resulting illegal copies have sufficiently high image quality and may be duplicated a plurality of times. Thus, it is clear that a superior technique of preventing illegal copying is much needed in the digital video industry.